Orthodontic brackets attach directly to teeth and serve to transmit corrective forces from an orthodontic archwire to the tooth to which the bracket is attached. The requirements for an orthodontic bracket are quite severe. First, it must have sufficient mechanical strength to withstand the forces to which it will be subjected, including the forces transmitted by an archwire, ligation forces, and mastication forces. Second, it must be chemically inert in the oral environment so that it will not corrode and will be and remain biologically inert. The bracket must meet these requirements, and still remain small enough to fit on the tooth.
The overwhelming majority of orthodontic brackets in use today are made of metal, usually stainless steel. Metal brackets meet all of the essential requirements, but they have one undesirable attribute--they are unsightly. A person undergoing orthodontic treatment has a conspicuous amount of metal in full view on the front surfaces of his or her teeth. And since the treatment extends over a number of years, this unsightly appearance must be endured for a considerable period of time.
The incentive to make brackets from less unsightly materials has existed for many years. But recently, orthodontic treatment has been given to increasing numbers of adults, for whom the unsightly appearance of metal brackets is more than a mere annoyance. Ceramic brackets have been proposed but have a tendency to stain, especially if the bracket must be worn for extended periods of time. Therefore, the incentive to provide more esthetic orthodontic treatment is even greater now than it has ever been.
Recently, sapphire (crystalline alphaalumina) has found commercial application as the material of construction for orthodontic brackets. U.S. Pat. No. 4,639,218 discloses sapphire orthodontic brackets, and sapphire orthodontic brackets are currently manufactured and sold by "A" Company, Inc., a subsidiary of the Johnson & Johnson Company, and Ormco Corporation. While these brackets are esthetically pleasing, i.e., are transparent and do not stain, concerns exist about the durability of the brackets made from sapphire under the considerable stresses on the bracket induced by the arch wire, ligation and mastication. Sapphire is more subject to fracturing and chipping than metal. Not only is this property undesirable from the standpoint of its use as an orthodontic bracket, but also, fractures and chips occurring during the fabrication of the bracket result in loss in yield.
Sapphire is characterized as having a number of crystalline planes. These planes include the "a" plane, "c" plane, "m" plane, "n" plane and "r" plane. It is generally known that the crystal structure is weaker along the "r" plane than along the remaining crystal planes, which remaining planes have approximately the same strengths.
The sapphire orthodontic brackets that have been commercially available are believed by us to be made from "r" plane blanks and thus an "r"-plane would generally be parallel to the front face of the bracket or from EFG technique grown material in which the edge face was an "m" plane and the "c" plane extended substantially perpendicularly between the front and back faces parallel to the longitudinal axis of the arch wire groove. The cleavage of the tie wings at the arch wire groove and/or chipping of the corners of the tie wings, especially during fabrication, can occur due to these orientations of "r" planes.
One solution to these problems with sapphire orthodontic brackets involves the use of specific orientations of the sapphire within the bracket as disclosed in our copending U.S. Pat. application Ser. No. 220,303, filed July 18, 1988.